1. Field of the Invention
The present invention relates generally to X-ray lithography apparatus and, more particularly, to an X-ray lithography source tube of the type comprising an electron beam source and a target for generating X-rays wherein the target is in the form of a composite target cone comprising at least an X-ray generating layer and a water-interface layer.
2. Prior Art
Large-scale integrated (LS1) circuits and, more recently, very-large-scale integrated (VLS1) circuits have been developed, among others, to cut costs. In the beginning, light in the visible range and ultraviolet radiation have been employed in a process known as photolithography. Soon thereafter, the electron beam came into widespread use in the manufacture of microelectronic circuits and masks because of its higher resolution than that obtainable with light. The higher resolution of the electron beam is at least partially due to the fact that it can be more precisely focused than can a light beam. With the need for still better resolution in the manufacture of VLS1 circuits, the use of X-ray radiation has been developed in a process known as X-ray lithography. See U.S. Pat. Nos. 3,742,229; 3,742,230; 3,743,842; 3,974,382; 3,984,680 and 4,287,235; each assigned to the Massachusetts Institute of Technology. The use of X-rays permits the use of much shorter wave lengths, typically below about ten angstroms (.ANG.), than those found in the ultraviolet region, typically about 3,000 to about 4,000 angstroms.
An X-ray lithography apparatus essentially comprises an X-ray generator, including a target, and a processing chamber. For a typical design of an X-ray generator, see the article "An Improved Annular-Shaped Electron Gun for an X-ray Generator" by J. L. Gaines and R. A. Hansen, Nuclear Instruments and Methods 126 (1975) pp. 99-101. As known, electrons, produced at the cathode by an electrically heated filament, are accelerated towards and strike the target. At the target, a small percentage of the energy of the electron beam is converted into X-radiation. The remaining, and much larger percentage of the energy of the electron beam striking the target is converted to heat, however. Hence, the target must be cooled, preferably by water, all the time that the X-ray generator is in operation. For a typical design of an X-ray generator featuring a target cooled by a high velocity flow of water, see the article "X-ray Lithography Source Using a Stationary Solid Pd Target" by J. R. Maldonado et al., J. Vac. Sci. Technol., Vol. 16, No. 6, Nov./Dec. 1979, pp. 1942-1945. As mentioned in this article, currently commercially available X-ray generators for use in lithographic systems have been plagued by maintenance and reliability problems. Some of these problems find their genesis in the shape, construction and choice of material for the target. These problems are continuing.
There is thus a need for an improved target for use in X-ray lithographic systems.